1. Field of the Invention
The present invention is directed generally to filters and more particularly to filters whose transmission characteristics may be varied.
Appparatus such as image intensifiers are used in a variety of applications including military, surveillance, underwater photography, and other applications where the amount of available light may be insufficient for the desired purpose. In U.S. Application Ser. No. 727,748, filed Apr. 26, 1985, and assigned to the same assignee as the present invention, it is suggested to use an imaging apparatus for intensifying the illumination reflected from a surgical site. The illumination reflected from the surgical site itself is on the order of one to five hundred footlamberts. However, the illumination reflected from objects surrounding the surgical site, such as cotton swabs, retractors, etc. may range anywhere from three hundred fifty footlamberts to two thousand nine hundred footlamberts. Because image intensifiers typically have a gain on the order of fifty thousand, when an image intensifier is adjusted to provide a proper image of the surgical site, the image intensifier may be damaged or burned out by illumination reflected by a retractor or the like. For this reason, image intensifiers have not heretofore been acceptable in surgical applications.